The present invention is generally directed to processes for the preparation of polymers and copolymers. More specifically, the present invention relates to polymerization processes which provide thermoplastic resin products which possess narrow polydispersity properties and which polymerization processes proceed with enhanced monomer to polymer conversion efficiencies. In particular, this invention relates to stable free radical mediated polymerization processes which provide homopolymers, copolymers, block, multiblock, graft, and the like, free radical initiated polymeric products with enhanced rates of polymerization and enhanced yields or monomer to polymer conversions.
The present invention provides, in embodiments, a polymerization process for the preparation of thermoplastic resin or resins comprising: heating a mixture comprised of a free radical initiator, a stable free radical agent, at least one polymerizable monomer compound, and a sulfonic acid salt compound, to form a thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity.
The present invention provides, in embodiments, homopolymers and copolymers with number average molecular weights (M.sub.n) above about 100 to about 1,000 and having a polydispersity or ratio of the weight average molecular weight (M.sub.w) to the number average molecular weight (M.sub.n) of from about 1.0 to about 2.0, and monomer to polymer conversions in excess of 50 percent.
The present invention provides, in embodiments, a pseudo-living polymerization process that enables the synthesis of narrow polydispersity homopolymers and copolymers, and thermoplastic resins from a variety of free radical reactive unsaturated monomers. The process, in embodiments, uses known free radical initiators in combination with a stable free radical agent, and a sulfonic acid salt compound.
In other embodiments, the stable free radical agent mediated polymerization processes of the present invention can be used to prepare block copolymers and multi-block polymer having narrow polydispersity properties thereby providing a means for preparing surface active or surfactant materials having well defined polydispersity and hydrophobelipophobe balance (HLB) properties.
The polymer resins produced by processes of the present invention, in embodiments, are essentially monomodal, that is the molecular weight distribution is narrow and indicative of a Poisson character and without shoulders or side bands. In embodiments, by repeating the heating step, comprising the combined initiation and polymerization step, there is provided a means for obtaining monomodal mixtures of polymer resins that are compositionally the same resin type having characteristics of both narrow polydispersity and known or selectable modality greater than 1. In embodiments, the process of the instant invention provides a means for conducting polymerization processes on multikilogram or larger scales. The aforementioned embodiments may be accomplished in a one or single pot reactor environment. In embodiments, although not wanting to be limited by theory, it is believed that the polymeric chain growth proceeds by a pseudoliving mechanism and can provide resins of variable molecular weights from very low to very high, for example, less than about 2,000 up to about 200,000 while maintaining narrow molecular weight distributions or polydispersities. In embodiments, block and multiblock copolymers can be synthesized by the aforementioned stable free radical moderated polymerization processes wherein each block formed is well defined in length by the sequentially added and reacted monomer and wherein each additional block that is formed also possesses a narrow molecular weight distribution.
In the aforereferenced commonly assigned U.S. Pat. No. 5,312,704, there are disclosed anionic polymerization processes which provide narrow polydispersity homopolymer and copolymer resins, and which resins possesses useful toner image properties, for example in forming gloss and matte images. The useful toner image properties are believed to be a function of the molecular weight properties and the narrow polydispersity properties of the resins. A salient feature of the anionic polymerization process is the relative speed and ease of the polymerization reactions which are typically conducted at lower temperatures to minimize deleterious side reactions. However, a serious disadvantage of such anionic polymerization processes, including cationic polymerization processes, used for the preparation of narrow polydispersity resins, block and multiblock polymers, is that these processes are highly sensitive to and unsuccessful in the presence of aqueous or protic solvent containing polymerization media, or in the presence of protonic or reactive functional groups.
In commonly owned and assigned U.S. Pat. No. 5,332,912 there is disclosed free radical polymerization processes for the preparation of a thermoplastic resin or resins comprising: heating from about 100.degree. to about 160.degree. C. a mixture comprised of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form the thermoplastic resin or resins with a high monomer to polymer conversion and a narrow polydispersity. A broad spectrum of free radical reactive monomers, including acrylate and acrylic acid derivatives, are suitable for use in the highly versatile polymerization process. The resins produced thereby possess useful toner image properties. Also disclosed are methods for improving the relative rate of the polymerization process including adding an inorganic acid, organic sulfonic or organic carboxylic acid during heating of polymerization reaction mixtures thereby increasing the rate of formation of the thermoplastic resin or resins from polymerization of the monomer or monomers. Another method disclosed for improving the polymerization process is accomplished by adding a tertiary amine promoter compound to the mixture thereby increasing or accelerating the rate of dissociation of the free radical initiator compound and thereby increase the apparent rate of the polymerization. Another method disclosed for enhancing the apparent rate of the polymerization reaction and for reducing the polydispersity properties of the polymeric products was achieved by including in the heated mixture a dialkyl sulfoxide additive, for example, dimethyl sulfoxide.
Notwithstanding the rate improvements achieved in U.S. Pat. No. 5,332,912, directed towards improving the efficiency and productivity of the free radical polymerization processes, there remains a need for further improvements in the time, and time dependent yield, of stable free radical mediated polymerization processes.
The present invention enables, in embodiments, the preparation of homopolymers, copolymers, block and multi-block copolymers using stable free radical moderated, free radical initiated polymerization systems which provide enhanced yields and accelerated, albeit thermally controlled, rates of polymerization in a variety of reaction media and in the presence of a variety of monomer and polymer functional groups.
The present invention provides, in embodiments, product resins with a latent, thermally reactive, functional group on at least one end which can be used for further reaction to prepare other resins with complex architectures. The present invention, in embodiments, provides polymerization processes that enable control of resin molecular weight, molecular weight distribution, modality of the products, and the like properties.
The present invention is directed, in embodiments, to pseudoliving stable free mediated polymerization processes which permit the economic preparation of homopolymeric and copolymeric narrow polydispersity resins with low, intermediate, or high molecular weights. The low molecular weight resins can be prepared without a chain transfer agent or molecular weight modifier which provides several advantages over conventional chain transfer mediated polymerization processes as illustrated herein.
Copolymers prepared by free radical polymerization processes inherently have broad molecular weight distributions or polydispersities, generally greater than about four. One reason is that most free radical initiators selected have half lives that are relatively long, from several minutes to many hours, and thus the polymeric chains are not all initiated at the same time and which initiators provide growing chains of various lengths at any time during the polymerization process. Another reason is that the propagating chains in a free radical process can react with each other in processes known as coupling and disproportionation, both of which are chain terminating and polydispersity broadening reaction processes. In doing so, chains of varying lengths are terminated at different times during the reaction process which results in resins comprised of polymeric chains which vary widely in length from very small to very large and thus have broad polydispersities. If a free radical polymerization process is to be enabled for producing narrow molecular weight distributions, then all polymer chains must be initiated at about the same time and premature termination by coupling or disproportionation processes must be avoided or eliminated.
The following patents are of interest to the background of the present invention, the disclosures of which are incorporated by reference herein in their entirety:
U.S. Pat. No. 4,581,429 to Solomon et al., issued Apr. 8, 1986, discloses a free radical polymerization process which controls the growth of polymer chains to produce short chain or oligomeric homopolymers and copolymers including block and graff copolymers. The process employs an initiator having the formula (in part) .dbd.N--O--X, where X is a free radical species capable of polymerizing unsaturated monomers. The molecular weights of the polymer products obtained are generally from about 2,500 to 7,000 having polydispersities generally of about 1.4 to 1.8, at low monomer to polymer conversion. The reactions typically have low conversion rates and use relatively low reaction temperatures of less than about 100.degree. C., and use multiple stages.
U.S. Pat. No. 5,059,657 to Druliner et al., issued Oct. 22, 1991, discloses a polymerization process for acrylic and maleimide monomers by contacting the monomers with a diazotate, cyanate or hyponitrite, and N-chlorosuccinimide, N-bromosuccinimide or a diazonium salt. The polymer produced can initiate further polymerization, including use in block copolymer formation.
In J. Macromol. Sci-Chem., A14(1), PP. 33-50 (1980) E. Rizzardo and D. H. Solomon, there is disclosed the acid induced free radical decomposition of hydroperoxides and the polymerization of methyl methacrylate with tert-butly hydroperoxide and sulfuric acid.
Other references cited in an international search report for the aforementioned U.S. Pat. No. 5,322,912 are: J. Am. Chem. Soc., 1983, 5706-5708; Macromol., 1987, 1473-1488; Macromol., 1991, 6572-6577; U.S. Pat. No. 4,628,019 to Suematsu et al., issued Aug. 10, 1986; U.S. Pat. No. 3,947,078 to Crystal, issued Aug. 10, 1976; and U.S. Pat. No. 3,965,021 to Clemens et al., issued Jun. 22, 1976.
The following references are also of interest: U.S. Pat. Nos. 3,682,875; 3,879,360; 3,954,722; 4,201,848; 4,542,182; 4,581,429; 4,777,230; 5,059,657; 5,173,551; 5,191,008; 5,191,009; 5,194,496; 5,216,096; 5,268,437; 4,546,160; 4,736,004; and 5,247,024.
In polymerization reaction processes of the prior art, various significant problems exist, for example difficulties in predicting or controlling both the polydispersity and modality of the polymers produced. These prior art polymerization processes produce polymers with high weight average molecular weights (M.sub.w) and low number average molecular weights (M.sub.n) resulting in broad polydispersities or low molecular weight (M.sub.n) and in some instances low conversion. Further, free radical polymerization processes of the prior art are generally prone to generating excessive quantities of heat since the polymerization reaction is exothermic. As the viscosity of the reaction medium increases dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in Principles of Polymerization, G. Odian, 2nd Ed., Wiley-Interscience, New York, 1981, page 272, the disclosure of which is entirely incorporated herein by reference. This is particularly the situation for reactions with high concentrations of soluble monomer, for example greater than 30 to 50 percent by weight soluble monomer, which are conducted in large scale reactors with limited surface area and limited heat dissipation capacity. Moreover, the exothermic nature of free radical polymerization processes is often a limitation that severely restricts the concentration of reactants or the reactor size upon scale up.
Further, gel body formation in conventional free radical polymerization processes may result in a broad molecular weight distributions and/or difficulties encountered during filtering, drying and manipulating the product resin, particularly for highly concentrated reactions.
These and other disadvantages are avoided, or minimized with the rate and yield enhanced polymerization processes of the present invention.
Thus, there remains a need for polymerization processes for the preparation of narrow polydispersity polymeric resins by economical and scalable free radical polymerization techniques and which polymers retain many or all of their desirable physical properties, for example, hardness, low gel content, processability, clarity, high gloss durability, and the like, while avoiding the problems of gel formation, exotherms, volume limited and multi-stage reaction systems, purification, performance properties of the polymer resin products, and the like, associated with prior art free radical polymerization methodologies.
The polymerization processes and thermoplastic resin products of the present invention are useful in many applications, for example, as a variety of specialty applications including toner and liquid immersion development ink resins or ink additives used for electrophotographic imaging processes or where monomodal or mixtures of monomodal narrow molecular weight resins or block copolymers with narrow molecular weight distribution within each block component are suitable for use, for example, in thermoplastic films and aqueous, organic, or mixed solvent borne coating technologies.